The Round Drum is available for the Thompson and the PPSh-41. It is unlocked after the Marksman III challenge for the Thompson and after the Marksman II challenge for the PPSh. It is the PPSh-41's most common attachment; its high rate of fire means the smaller magazines need frequent reloading; and also, the attachment is unlocked at the Marksman II challenge as opposed to the Marksman III challenge, meaning it is easier to unlock.

The increase in magazine size that is given depends on the gun. Generally, it increases magazine size by 50%, but with some weapons it can increase magazine size by a greater amount. The attachment does not increase starting ammo capacity unless paired with Scavenger Pro. Light machine guns benefit greatly from the extended magazine as their already large magazine is doubled, and combined with their high penetration allows the player to sustain good suppressive fire. Extended Mags is less useful with assault rifles or SMGs, because most players tend to reload after each kill anyway, though it can be useful if the player does a lot of spraying. The P90 becomes akin to a light machine gun with Extended Magazines due to its already large 50 round magazine being turned into a 75 round magazine, which allows the user to hold down choke-points, but has a much quicker reload than the LMGs. While using Extended Magazines on the P90, a player can sprint for a longer period than normal (patched on PS3/X360, still possible on PC). This allows users to equip another first tier perk instead of Marathon like Sleight of Hand or Bling. The P90 is the only weapon which gains this advantage. The AA-12 in particular has a very small magazine by default, so Extended Mags is an extremely popular attachment for it, especially due to the fact that it doubles the capacity to 16 shells.

Extended Mags return in Call of Duty: Black Ops, and can be used on most weapons. This attachment is not to be confused with Dual Mags, which is another attachment in Call of Duty: Black Ops that decreases reload time by taping two magazines together.

In singleplayer mode, the attachment increases the maximum ammunition count, by having in reserve the same number of extended magazines as the standard magazines. For example, the standard M16 has 660 rounds in reserve (22 magazines of 30 rounds), while the one with Extended Mags has 990 rounds (22 magazines of 45 rounds). Extended Mags in campaign can be found on the M16, M14, Galil, AK47, Commando, Skorpion, MAC11, AK74u, Uzi, PM63, Kiparis, HK21, RPK and M60, and are always used on the MP5K and the Stoner63.

The multiplayer iteration of Extended Mags doesn't increase maximum ammunition, except on the Wii version. Unlike in Call of Duty: Modern Warfare 2, the magazine model changes when Extended Mags are used, just like in Call of Duty: World at War. A notable example of this are the HK21 and RPK, where they will be given drum magazines and be called "HK21 Drum Mag" or "RPK Drum Mag." The M60 is also given the name "M60 Big Ammo." Other weapons will be shown using longer magazines. The exception to this are the M14 and the M60, which do not use a different magazine model.

Interestingly, if Extended Mags is equipped on the AK47 or the AK74u, their magazine model becomes the one regularly used by the RPK: a large, rust-colored Bakelite plastic magazine, though they get 45 rounds instead of the RPK's 40-round capacity.

Due to balancing purposes, a weapon cannot accept Extended Mags and Dual Mags at the same time by using Warlord. Rapid Fire cannot be paired with Extended Mags or Dual Mags either.

The Extended Mags returns in Call of Duty: Modern Warfare 3. It increases magazine capacity, and now in multiplayer it increases the reserve ammmo count as well, in the same way as the singleplayer mode of Black Ops. It mostly increases magazine capacity by 50%.

Extended Mags also briefly appear in Special Ops mission mode, being used on the starting M4A1 in Smack Town and the SCAR-L in Light 'Em Up. In these appearances however, the reserve ammo count is not increased, and the HUD doesn't mention this attachment.

Prior to the January 25th update, there existed a bug for Extended Mags that when equipped on a Shotgun, the weapon would shoot 50% more pellets per shot, effectively increasing damage and consistency in addition to the standard effects of the attachment. When the Extended Mags attachment was combined with the shotgun-exclusive DamageProficiency, it would produce a staggering 210% increase in the damage output of the base version. It has now been patched.

It is most useful for weapons that have low ammo capacities since it increases magazine size as well as starting ammunition. This can give a player the option to choose a different tier 1 perk. As with this attachment, an average player will usually have enough ammo to last their life without Scavenger.

As well, weapons that are less diverse, such as an SMG, Shotgun or a Sniper rifle, are quite often seen with Extended Mags in use. Weapons that have many attachment options, such as an Assault Rifle, or weapons such as an LMG that already have huge magazines, see much less use of this attachment, because many players will find that something such as an optical attachment, an underbarrel weapon, etc. will be of more use.

Extended Mags returns in Call of Duty: Black Ops II as Extended Clip; it increases the magazine's capacity like before. However, unlike the previous games, the capacity is increased by lower than 50%. Fortunately, the attachment still gives extra ammo, for both singleplayer and multiplayer modes, as well as Zombies. Now, the attachment increases magazine capacity by around 33-35% for most weapons, but has a downside of increased reload times by 10%.

It was planned to be a perk in singleplayer instead of an attachment, as a singleplayer perk icon of it can be found in the game files.

The Extended Clip attachment is now featured in the Zombies map Origins, equipped on the AK74u and the B23R when obtained via the Mystery Box. Here the attachment increases the capacity by a higher amount than in the singleplayer and multiplayer modes. While the B23R's reserve ammo is also increased in the same way as in previous games (same number of magazines in reserve), the AK74u with Extended Clip has its own ammo count instead.

Extended Mags return in Call of Duty: Ghosts, available in multiplayer and Extinction. It increases magazine capacity by 50%, and increases maximum ammunition as well in both game modes. Due to the consistent number, many large-magazine weapons will find that this attachment very beneficial to their arsenal; however, weapons that have small magazines, such as the Bulldog and the Remington R5, will find constant use of this attachment; and the popularity of it is easily reflected by the players.

In campaign mode, the attachment is used in the missions "Captured" and "Terminus", and is available on the Atlas 45, the SN6 and the AMR9. Due to Mitchell's disabled prosthetic left arm in these two missions, reserve ammunition cannot be carried for any weapon, which much be swapped for another one once emptied.

While Extended Mags normally doesn't change the appearance of the weapons, the ASM1 Speakeasy and Royalty variants (which have this attachment integrated) have a drum magazine rather than the standard box magazine.

In Zombies, upgrading most weapons via the Pack-a-Punch machine grants them the Extended Mag attachment, as evidenced by the changed magazine models and reloading animations. It increases magazine capacity by a higher amount than in other modes, but doesn't always increase reserve ammunition.

Extended Mag returns in Call of Duty: Infinite Warfare. The attachment is only available to ballistic weapons, and usually increases their magazine capacity by 50%. Extended Mag also increases the reserve ammunition in campaign, multiplayer and Zombies.

One thing to note is that while the base Volk assault rifle is an energy weapon, the Retro variant is ballistic, which means the latter accepts the extended mag attachment.

Infinite Warfare - Ammo Per Magazine

Note: regarding the values in Zombies, the first one is for the base weapon, the second one is forthe Pack-a-Punch upgrade, and the third one is for the double Pack-a-Punch.

All LMGs (except the AUG HBAR) with Extended Mags in Modern Warfare 2 have so many rounds in one magazine that on the HUD, the bullet count overlaps the white bar that separates ammo and equipment indicators. This does not occur with resolutions higher than 1280x720, which are only available on PC.